build_kernel_image: use a salt for verity.
build_kernel_image: use a salt for verity.

BUG=chromium-os:12138
TEST=Adhoc
TESTED_ON=Kaen
Build an image. Look for "Generating root fs hash tree (salt <foo>)." in the
output. Boot the image, grep for 'salt=' in dmesg. All should be well.

Change-Id: Ia7d8504033ff1e62ba451129be5796702809cc7c
Signed-off-by: Elly Jones <1693f95ea06ee6d7b2276cb599c3f4f33a951d9c@chromium.org>
Reviewed-on: http://gerrit.chromium.org/gerrit/7335

andrejro/coreos--scripts,andrejro/coreos--scripts,andrejro/coreos--scripts,endocode/scripts,sigma/coreos-scripts,coreos/scripts,mischief/scripts,bcwaldon/coreos-scripts,BugRoger/scripts,vmware/coreos-scripts,nicescale/scripts,vmware/coreos-scripts,BugRoger/scripts,marineam/coreos-scripts,smilart/scripts,fivethreeo/scripts,smilart/scripts,mischief/scripts,fivethreeo/scripts,zhang0137/scripts,trnubo/scripts,trnubo/scripts,mjg59/scripts,bcwaldon/coreos-scripts,fivethreeo/scripts,bcwaldon/coreos-scripts,gtank/scripts,BugRoger/scripts,gtank/scripts,crawford/scripts,marineam/coreos-scripts,marineam/coreos-scripts,glevand/coreos--scripts,sigma/coreos-scripts,gtank/scripts,glevand/coreos--scripts,zhang0137/scripts,nicescale/scripts,mischief/scripts,nicescale/scripts,endocode/scripts,coreos/scripts,crawford/scripts,endocode/scripts,mjg59/scripts,mjg59/scripts,zhang0137/scripts,vmware/coreos-scripts,sigma/coreos-scripts,trnubo/scripts,smilart/scripts,crawford/scripts